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09 - 09 - 2017 Yet another rough diamond imitation analyzed by our research department

On July 14, 2017 we reported on 2 topaz crystals cut to imitate rough diamonds. Various methods of examination clearly indicated that the heavily-included stones were topaz and not diamond. Last week, two more rough stones were submitted to us. This time the stones ‘felt’ like rough diamonds, and even diamond testing pens (based on thermal conductivity) pointed to diamonds. But the density measurements clearly indicated that these were again imitations; more precisely, moissanite. The stones had been cut to resemble rough diamond, weighing each 7.14 and 5.01 carats, and have a slight greenish colour. 

Moissanite is an important diamond imitation on the market, usually used as a substitute for polished diamonds. The US company, C3 Inc., developed and put it on the market in 1997. Since then, many manufacturers entered the market. Natural moissanite is extremely rare and appears in relation to iron-nickel meteorites and certain rare ultramafic-depth stones. 

Synthetic moissanite, being silicon carbide, has fooled many experienced jewellers with its very high hardness (9.25 on the Mohs scale) and high refractive index (2.6 – 2.7). Synthetic moissanite has a very high thermal conductivity, which has rendered some tests useless. To our knowledge, natural moissanite has never been found in significant sizes and of good quality.

The table below shows how the physical, chemical, and optical properties of moissanite and topaz differ from those of diamonds, and how these differences help us to distinguish them from diamonds:

 

 

Diamond

Synthetic
Moissanite 

Topaz

Comment

Specific Gravity

3.52 

3.22

3.53

3D scanners used in the diamond industry (e.g. Sarin or Ogi) can be used to differentiate synthetic moissanite from diamonds

Thermal conductivity

Very high

Very high

Medium

Diamond testers based on heat conductivity (e.g. Preasidium) are used to differentiate topaz from diamonds

Mohs hardness

10

9.25 - 9.50

8

Topaz clearly shows marks using a Borazon hardness pen

Refractive index

2.42

2.648 – 2.691

1.619 - 1.627

No reading possible for diamond and moissanite using a standard gem refractometer with refractive liquid of 1.80

Birefringence/doubling

None

0.043 (strong doubling)

0.008 - 0.010

 

Crystal system

Cubic

Hexagonal

Orthorombic

 


The two stones discussed here should certainly set off the alarm bells of any experienced rough diamond trader or polisher that would encounter them. Problems may arise when stones such as these, or smaller sizes of polished moissanite, are mixed into rough diamond parcels.

 

Source: HRD Antwerp
 

 
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Vicenzaoro September
日期 : 2017年9月23日 - 9月27日
地點 : Hall 3.1, International Pavilion, Fiera di Vicenza, Vicenza, Italy
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